1. Field of the Invention
The present invention relates to a planar transformer to be used for various types of circuits such as power supply circuits and inverter circuits for fluorescent tubes.
2. Description of the Related Art
Recently, miniaturization and lightening of all of the electric devices have been strongly required. However, magnetic parts such as inductors and transformers among component parts of electronic circuits have been less miniaturized and lightened than semiconductor elements, resistances and condensers, which becomes a serious reason for preventing the miniaturization and lightening of circuits. Although these magnetic elements are particularly indispensable for various kinds of power inverter circuits, it is difficult to miniaturize and lighten them. In an electronic automatic exchange, for example, many dc-to-dc converters are used in every electronic circuit board in accordance with desired supply voltage, and magnetic elements occupy much of the whole volume of these dc power supplies. The magnetic elements are also used for a back light in the fluorescent tube type utilized for a liquid-crystal display of a word processor or personal computer, and therefore it is indispensable to make magnetic elements thin for a thinner display.
In this circumstance, planar inductors or transformers are greatly developed. When a planar transformer is manufactured, it is necessary to sufficiently increase magnetic coupling between primary and secondary coils in order to efficiently transmit a signal or power from a primary coil to a secondary coil. In other words, it is necessary to design a transformer to make coupling coefficient between both coils as close as possible to 100%. The coupling coefficient k of the transformer is indicated by the following formula EQU k=.PHI..sub.21 .PHI..sub.1,
where, .PHI..sub.1 indicates magnetic flux produced by the primary coil, and .PHI..sub.21 indicates magnetic flux which interlinks a secondary coil in the magnetic flux produced by the primary coil.
Provided that a resistance component is negligible, a primary-to-secondary ratio of voltage is proportional to a product of k and the ratio of winding numbers, and signal transmission is completely performed when k is unity. Provided that various kinds of loss are negligible, efficiency of power transmission from a primary coil to a secondary coil is proportional to square of k. Therefore, slight reduction of coupling coefficient causes remarkable reduction of efficiency of power availability. Thus, increasing of coupling coefficient of a transformer is much important for the performance of the transformer.
In general, in order to increase coupling coefficient of a transformer, it is necessary to make a path of magnetic flux produced by a primary coil correspond to that of magnetic flux produced by a secondary coil as much as possible. However, since the distribution of magnetic flux is complicated in the case of a planar transformer, it is difficult to completely realize this condition. Nowadays, there is no unified way of design regarding a method of arranging primary and secondary coils, and this has been decided on the basis of trial and error. When a primary coil remarkably differs from a secondary coil particularly in the winding numbers and sizes, an optimum method of designing coils is not quite obvious from the viewpoint of the performance of the transformer. Accordingly, a planar transformer having a sufficiently high coupling coefficient has not yet been realized for this reason.
As described above, it is expected that a planar transformer will contribute to miniaturization and lightening of electronic circuits. However, a designing method for increasing coupling coefficient has not yet been known so that the planar transformer remains far from practical use.